The peritoneal cavity is formed by the fourth embryonic week and is derived from the mesoderm.2
The abdominopelvic cavity is lined with a thin continuous layer of peritoneum. The cavity is completely sealed in males but communicates with the external environment via the fallopian tubes in females. Peritoneum that envelops the organs is referred to as visceral peritoneum,
and the peritoneal layer that lines the walls of the abdominopelvic cavity is referred to as parietal peritoneum
. This thin layer coating all surfaces of the peritoneal cavity and its organs secretes a small amount of serous fluid, approximately 50 mL, which acts to lubricate visceral surfaces, allowing them to move without friction.1
As organs develop along the posterior abdominal wall and protrude into the peritoneal cavity, they are covered by visceral peritoneum except at their hilum, where blood vessels, nerves, and lymph enter and exit the organ. The hila of peritoneal organs are considered bare areas because they lack a peritoneal covering. These bare areas are part of the retroperitoneum.
Divisions of the Peritoneal Cavity
The peritoneal cavity is generally divided into two compartments: the greater sac and the lesser sac. The greater sac
is the largest, housing the liver, spleen, stomach, first portion of the duodenum, jejunum, ileum, cecum, transverse colon, sigmoid colon, and the upper two-thirds of the rectum.1
This large sac contains several potential spaces that must be evaluated for free fluid.
The lesser sac may be thought of as a diverticulum of the greater sac and is also referred to as the omental bursa by some. The lesser sac does not contain any organs. This potential space lies immediately posterior to the stomach, extending superiorly to the left suprahepatic recess between the posterior left lobe of the liver and the left hemidiaphragm.
FIGURE 5-1 Addison lines. Nine regions of the abdominopelvic cavity based on Addison lines. The abdominal cavity is divided into nine regions by two sagittal midclavicular lines and two axial lines, the transpyloric line, and the transtubercular line.
FIGURE 5-2 Quadrants of the abdominopelvic cavity. The abdominal cavity can be divided into four quadrants: right upper quadrant, right lower quadrant, left upper quadrant, and left lower quadrant.
The lesser sac extends inferiorly into the fold of the greater omentum. This may also be referred to as the inferior recess of the lesser sac
or omental bursa
. Note that this fold is patent in infants and small children but generally fuses in adults, thereby significantly limiting the caudal extent of the lesser sac (Fig. 5-3A, B
). The lesser sac’s anterior wall is formed by the posterior stomach, whereas superiorly it is enclosed by the lesser omentum, also called hepatogastric ligament
. The splenorenal and gastrosplenic ligaments create the left lateral wall of this pocket (Fig. 5-4
). The omental foramen, also called the foramen of Winslow
, is located at the right lateral aspect of the lesser sac and is the only opening communicating with the greater sac. This opening is found posterior to the hepatoduodenal ligament
, the thickened right border of the lesser omentum that guides the portal triad into the liver (Fig. 5-4
The lesser omentum—also called the small omentum
, gastrohepatic omentum
, and gastrohepatic ligament
—is a fused double layer of peritoneum stretching between the lesser curvature of the stomach and the left sagittal fissure for the ligamentum venosum (transverse fissure). This ligament creates the anterior superior border of the lesser sac, separating it from the supracolic compartment of the greater sac (Fig. 5-5
Within the greater sac is a large apron-like double-layered sheet of peritoneum called the greater omentum
that extends inferiorly from the greater curvature of the stomach and transverse colon. The greater omentum extends inferiorly, anterior to the bowel, folds inward, and travels superiorly to attach on the transverse colon. The anterior and posterior adjacent layers are separate in infants but typically fused
in adults and contain a variable amount of fat3
). The greater omentum functions to prevent the parietal peritoneum of the anterior abdominal wall from adhering to the visceral peritoneum. This mesenteric drape is very mobile and moves to areas of inflammation, surrounding the inflamed area by creating adhesions to wall off infection (Fig. 5-6A, B
). It also acts to cushion the abdominal organs to prevent trauma and acts to prevent the loss of body heat from abdominal organs.
FIGURE 5-3 Lesser sac—schematic sagittal sections, lateral view. Note the patent inferior recess of the lesser sac in the infant (A) and the fused nature of the same space in the adult (B). The red arrow indicates the omental foramen.
FIGURE 5-4 Omental foramen. The black arrow extends through the omental foramen through the width of the lesser sac. The opening is posterior to the hepatoduodenal ligament. The lesser space is seen immediately posterior to the stomach. P, peritoneal cavity.
The greater omentum subdivides the greater sac into a supracolic (above the colon) compartment and an infracolic (below the colon) compartment. The supracolic compartment is located anterior to the greater omentum and stomach and inferior to the liver. The infracolic compartment is located posterior to the greater omentum, surrounding the small bowel and colon within the remainder of the greater sac (Fig. 5-7
). This division is important because it limits the spread of infected materials, pus, ascitic fluid, and malignant
cells within the peritoneal cavity. Communication between these two compartments is via the paracolic gutters, the lateral borders of the ascending and descending colon.
FIGURE 5-5 Lesser omentum. The lesser omentum is a double layer of peritoneum that stretches between the lesser curvature of the stomach and the left sagittal fissure for the ligamentum venosum.
FIGURE 5-6 Greater omentum. A: Longitudinal image in a patient with appendicitis demonstrates hyperechoic omental fat (arrows) surrounding the inflamed appendix. B: Transverse image again demonstrates the hyperechoic omental fat (arrows) seen surrounding the inflamed appendix. (Images courtesy of Ultrasound-Cases.info, owner SonoSkills.)
Potential Spaces of the Peritoneum
As organs grow into the peritoneal cavity, several pockets and recesses are formed by the organs, their vascular connections, and suspensory ligaments, thereby creating a complex landscape for sonographers to examine when performing abdominal and pelvic examinations. Ligaments divide portions of the peritoneal cavity. Sonographers require a working knowledge of these ligaments to understand where to look for fluid within the peritoneal sac and how to image and describe its location. See Table 5-1
for a description of the ligaments of the peritoneal cavity.
FIGURE 5-7 Divisions of the peritoneal cavity. Green represents the supracolic compartment of the greater sac; pink represents the infracolic compartment of the greater sac; and blue represents the lesser sac.
TABLE 5-1 Ligaments of the Peritoneal Cavity
Also called the lesser omentum, smaller omentum, and gastrohepatic omentum, it connects the lesser curvature of the stomach and the left sagittal fissure for the ligamentum venosum (transverse fissure) of the liver.
Thickened free edge of the lesser omentum through which courses the portal triad; it connects the liver to the duodenum.
Double-layered fold of peritoneum that ascends from the umbilicus to the liver; contained within it is the ligamentum teres. The falciform ligament passes onto the anterior and then the superior surface of the liver before splitting into two layers. The right layer forms the upper layer of the coronary ligament; the left layer forms the upper layer of the left triangular ligament.
Bifurcation of the falciform ligament layers that fuse with the parietal peritoneum to form borders of the bare area of the liver, suspending the liver from the diaphragm. The right branch becomes the coronary ligament and the left branch becomes the left triangular ligament, limiting the greater sac at its cephalad extent into anterior and posterior compartments in the right subphrenic area.
Left triangular ligament
Formed by the left branch of the falciform ligament and the parietal peritoneum, it forms the left extremity of the bare area of the liver.
Also called the lienorenal ligament, it connects the splenic hilum to the posterior abdominal wall, through which the splenic vein and artery travel.
It connects the stomach to the spleen and inferior diaphragm.
A suspensory ligament that extends from the lateral uterine sidewalls to the pelvic sidewalls, dividing the pelvis into anterior and posterior compartments in the female
Remnant of the fetal umbilical vein, which is contained within the falciform ligament; it passes into a fissure on the visceral liver surface to join the left branch of the portal vein in the porta hepatis.
It exhibits as a fibrous band (remnant of the ductus venosus) attached to the left branch of the portal vein. It ascends in a fissure on the visceral liver surface to attach above the inferior vena cava. In fetal circulation, oxygenated blood flows to the liver via the umbilical vein (ligamentum teres). Most of the blood bypasses the liver via the ductus venosus (ligamentum venosum) and enters the inferior vena cava.
Potential spaces are areas created by the peritoneal layer, reflecting between two organs or an organ and the peritoneal wall (typically posterior). A potential space is an empty fold; however, when disease is present, fluid or other materials may collect in this space. Because many pathologies present with excretions (ascitic fluid, blood, pus) into the peritoneal cavity, sonographers must examine these potential spaces and characterize the fluid as part of the abdominal and pelvic examinations. The following text includes anatomic descriptions of each major potential space of the peritoneal cavity.
Left Anterior Subphrenic Space
The left anterior subphrenic or suprahepatic space is an extension of the greater sac between the diaphragm and the anterior superior liver leftward of the falciform ligament.
Left Posterior Suprahepatic Space
The left posterior suprahepatic space is also called the superior recess of the lesser sac
; this space is an extension of the lesser sac between the diaphragm and the posterior superior liver. See Figures 5-3
Right Subphrenic Space
The right subphrenic or suprahepatic space is an extension of the greater sac between the right hemidiaphragm and the anterior superior liver rightward of the falciform ligament (Fig. 5-8
FIGURE 5-8 Subphrenic spaces. A: Right anterior subphrenic space and hepatorenal space. Longitudinal image of the right upper quadrant demonstrates fluid within the right anterior subphrenic space (single arrow); ascites is also seen within the hepatorenal space (double arrows). B: Right anterior subphrenic space and left anterior subphrenic space. Transverse image of the epigastrium demonstrates fluid within the right (*) and left (**) anterior subphrenic spaces separated by the falciform ligament. LLL, left lateral lobe of liver; LML, left medial lobe of liver; P, pancreas; SPL, spleen. (Image A: Courtesy of Philips Medical Systems, Bothel, WA.)
The hepatorenal space is also referred to as Morrison pouch
. This peritoneal potential space is created by the peritoneum, reflecting from the liver over the right kidney and right posterior peritoneal wall. When the patient is in a supine position, this space is the most gravity-dependent potential space of the abdominal cavity, collecting fluid from the supracolic area and the lesser sac. See Figure 5-8
The omental bursa, or lesser sac, is sandwiched between the posterior stomach and parietal peritoneum covering the anterior pancreas (front to back) and the splenorenal and gastrosplenic ligaments and epiploic foramen (side to side). In cases of posterior gastric wall perforation or inflammation or trauma to the pancreas, fluid or a pseudocyst may be identified in this space (Fig. 5-9
FIGURE 5-9 Lesser sac. A: Transverse image of the epigastrium demonstrates a hematoma (arrows) within the lesser sac in a patient with acute pancreatitis. The posterior wall of the stomach (ST) borders the hematoma anteriorly. The pancreas (PANC) forms the posterior border. B: The computed tomography scan shows the large fluid collection with debris in the omental bursa in a patient with pancreatitis. (Images courtesy of UltrasoundCases.info, owner SonoSkills.)